Telephone company personnel have said that switch contact maintenance constitutes 50 to 75 percent of central office exchange maintenance.
The type of switch to be protected is called, in telephone terminology a "spring pile-up arrangement", operating in indirect response to a telephone dial and controlling a stepping solenoid, the inductive cost of which is a source of voltage spikes injurious to switch contacts. The term "inductive coil" is telephone terminology to distinguish from a load coil.
Stepping solenoid coils from various manufacturers have varied in their resistances by as much as from 40 to 1800 ohms. The high resistances reduce switch contact current to very low levels as creates special problems and a versatile switch protector circuit has been greatly needed.
Every six months a technician has been required to use extensive instrumentation--operating into thousands of test jacks on the spring-pile-up switches. It was necessary to test each of these switches individually to determine whether the switch was performing to specifications--then fine tuning was required of that switch in order to operate within its prescribed limits of resistance. The switch had to be tuned in such a manner as to operate into the required resistance in a subscriber line.
By use of my invention, as later described, it has been found that it is no longer necessary to fine tune the switches at all. But a single transistor for switch protection will fail, in most telephone exchange uses, causing costly down-time.
When there is a more rapid pulse rate, such as 30 pulses per second, together with high coil resistance in the inductive coil, then a single transistor device will not stand up.
In the prior art, the proposals for the protection of switches in telephone circuitry have included a single transistor circuit, such as shown in the U.S. Pat. No. 3,912,941, issued Oct. 14, 1975, to Thomas M. Passarella, and titled: ISOLATION CIRCUIT FOR ARC REDUCTION IN A DC CIRCUIT.
The single transistor in U.S. Pat. No. 3,912,941 is not protected from thermal failure, from overheating in series with the inductive coil.
In addition to the heating effect that will destroy the transistor, and even before it is destroyed, there are also current problems through the inductive coil, because the transistor is acting as a resistor, and it is reducing the line current from the switch through the coil.
I have discovered that my two-transistor circuit with its control transistor protects the series transistor and prevents overheating, but it also has the effect of maintaining the current through the coil at either zero or else at a specific value--it should not be anything inbetween.
In the past it has been necessary to test switch circuits to see if they will reach the operating limit of the switch, since some switches are 1200 and some are 1900 ohms, for example. But, the surprising discovery with the circuit of this invention is that, even a light contact operation will still give a full current-carrying capacity, hence versatility of use.
Heretofore a light contact operation would cause a chattering switch, one non-functional for carrying a substantial current.
A high coil load on a one-transistor circuit will cause spike trouble, a high resistance coil might be beyond the gain limits of the transistor. In other words, the ratio between the current through the collector to emitter on the one hand, and the base current on the other hand, which is the gain, is too high when the coil is of high resistance, causing thermal failure of the transistor from heating.
A two-transistor cirucuit has never before been used to my knowledge to protect a mechanical switch and, for most telephone exchange repair work, a mere two-transistor circuit will fail to solve certain practical problems as I will explain.
However, my special two-transistor circuit has the advantage that, even with higher coil resistances, the transistor that is in series with the solenoid coil is fully turned on.
One of the important features of my special circuit is that no matter what resistance of the load is, even if the inductive coil load were varied, even so high as to cause a 5-ampere current at the load transistor, yet the current at the switch contacts would be very low, such as eleven milli-amps.
To use such low resistance coils, heat sinks would be needed on the transistor to reduce the current. But by putting heat sinks on the protective circuit, then loads of 5-amps and above can be handled at the transistors, whereby coils of lesser resistance can be handled, also.
But I have also discovered that the use of a mere two-transistor circuit will not work to provide a fail-safe operation in the practical problems involved, and I use a varistor to protect the transistor circuit.
The varistor carries high voltage transients generated in the inductive coil to ground at a voltage lower than the collector-to-emitter breakdown voltage of the transistor. This is important because telephone solenoid coils can cause spikes as high as 100,000 volts.
I have discovered that a silicon carbide varistor can cause faulty operation, probably because it permits leakage current therethrough and that a metal oxide varistor will protect both of the prtoective transistors from current leakages.
Economy is gained, I discovered, by using two common inexpensive transistors in the protective circuit, rather than one carefully selected and more expensive transistor, the economy being an additional benefit above the other discussed advantages.
A particular objective of this invention is to prevent expensive "down-time" on equipment.
Another objective is to provide a means of reducing maintenance on existing equipment which has mechanical switches operating in air by installation on already burned switches and cause them to be useable, as has been proven possible.
It is an object of this invention to provide 100% suppression of contact arc and elimination of tiresome noise in telephone exchanges caused by arcing, giving quiet, peaceful switching; and preventing also such noise from being impressed upon subscriber lines.
The top voltage the coil can generate is held down by the varistor to whatever is the voltage rating of the varistor.
A coil could generate voltage as high as, for example, 1000 volts if the varistor were not in the circuit. But, with a varistor being in the circuit, the coil will generate only as much voltage as the varistor allows it to generate.
In a transistor circuit, without a varistor, the transistor allows the coil to generate however much voltage the rating of the transistor will permit before the transistor breaks down.
The circuit of this invention provides the following advantages:
(1) Doesn't blow up the transistor. PA1 (2) Doesn't slow down or "slug" the coils, the circuit is not slowed down; but, in some prior art installations, this does happen. PA1 (3) Extreme versatility. Works in any such uses. PA1 (4) Reduces noise, by eliminating arcing; total noise is cut by as much as 70%. PA1 (5) Self-protective.
To further explain, a slowed pulsing switch operates fewer times per second, for example, perhaps 20 pulses per second, instead of 30 pulses per second. This is very serious because, in some circuits, while the telephone dial operates at 10 pulses per second, the switch may be forced to operate at 10 pulses between digits on the dial. In other words, the switch goes step, step, step, with each digit on the dial. But when the switch gets to a certain point, it may be required to automatically operate 10 steps between one digit on the dial and the next digit on the dial. This is because the switch is hunting for a vacant line in the telephone system and, if you slow down that switch, then it will not make its required 10 steps before the dial reaches the next digit. This causes the telephone call to not go anywhere. All the caller will hear in the telephone is the beeping tone and he will need to re-dial the call. If in the second dialing he hits the same switch, he will have the same negative results, the same beeping and a call failure again. So it is very important that the circuit not be "slugged", in other words, that the circuit not be slowed down by the introducing in the circuit of a protective device that would slow the switch down.
An advantage of this invention is that the device introduced to protect the circuit will not slow the switch down.
Another cause of difficulty in the prior art is, if a switch will handle 1000 calls per hour, then if that switch is slowed down by as little as 10% so that it will handle only 900 calls an hour, then this results in major disappointments causing telephone officials to be enraged. They had plans to handle 1000 calls with the equipment and 100 of the calls can't even get in. Those answers would not even get a dial tone.
The circuit of this invention likely in use may be subjected to as many as 200,000,000 pulses over a 3-month period because of the coil that really damages an unprotected switch operating in air.
It is a further object of this invention that ohmic matching between subscriber lines and the telephone exchange shall in no manner be unfavorably affected or disturbed by this invention.
It is another object of this invention that a timing square wave shall be faithfully preserved throughout this circuitry, from initiation of the wave as a dial pulse, until complete utilization of the wave has occurred, with no measurable or discernible harmful distortion introduced by the invention.
An objective is to reduce telephone subscriber rates by reducing switch maintenance and replacement costs and by providing, in one easily-installed molded jacket, a complete switch protector of versatile use, eliminating need for very costly special units for each variation in need, and eliminating high inventory costs and problems and ordering problems for quantities of non-versatile units, all this being made simple.
Particularly, it is an objective hereof that required constant current in a stepping-switch inductive coil shall by no means be disturbed due to use of this invention, but shall be preserved.
An important objective is that inductive coil slugging is eliminated due to maintenance of a high enough coil voltage of the inductive coil.
Low currents at switch contacts such as below 5 milli-amps, cause contact oxidation and resistance build-up in the contacts and switch failure, and switches carrying currents above 5 milli-amps are self-cleaning. However, the use of parallel circuits with a wire with an extra resistor in parallel with two resistors in my circuit to get more than 5 milli-amps of current to the switch was unobvious to me for many, many months, and it is a feature not found in the prior art U.S. Pat. No. 3,912,941.
The parallel resistor circuit is extremely necessary when coils in the circuit are above 100 ohms. There are many coils in telephone equipment that are 375 ohms and many even at 1000 ohms, although a lesser percentage of them cause arcing problems.
A further objective is to provide a protector causing the switches to operate more smoothly and quietly, surprisingly, for mechanical reasons also, the main mechanical reason being that there is no oxidation build-up on the contacts interferring with smooth operation, hence achieving further quiet operation.
I have discovered that wave form distortion of dial pulse typical to the prior art is eliminated by this invention due to use of a metal oxide varistor as opposed to silicon carbide varistors of the prior art, and due further to the single device operation of said varistor with a two-transistor circuit. Inductive coil voltage is held high by use of the two-transistor circuit, thus preventing low-voltage discharge and preventing coil slugging. Furthermore, timing of the square wave is preserved intact. The metal oxide varistor operates independently of the inductive coil current, whereas, in the prior art, silicon carbide devices caused clipping of wave form, having voltage varying with current. In telephone embodiment, dial pulses typically constitute an amplitude of 48 volts, a duty cycle of 60% and a repetition rate of up to 30 pps. This invention has performed successfully with its two transistors, not only at reprates of 30 pps., but at a multiple thereof of an order of magnitude of 0.5(10)2. Whereas a single transistor circuit has performed successfully at such rep rates as 10 pps., and in some instances at much higher rep rates, the two-transistor circuit of this invention is known by the experimental determination of the inventor to be reliably successful at such typical rep rates as 30 pps. and higher.
In a most rigorous and comprehensive longevity study performed upon the invention by a significant public electrical utility cooperative, this invention was found to have a longevity of 25 years, an order of magnitude of 0.5(10)2 of gain over typical six months maintenance schedule of contacts. From this longevity study, it was determined that contact inspection and fine tuning were not required during the accelerated 25-year test. Furthermore, incorporating the invention in telephone exchange equipments under experimental cognizance of the inventor, it was found that contacts so badly pitted as to be due for replacement were reinstated for indefinitely prolonged longevity upon installation of the invention, eliminating need for replacement and likewise eliminating need for subsequent inspection and fine tuning of pitted contacts, resulting performance being equal to that of new contacts.
In the art, and particularly in telephone embodiment, a plurality of manufacturers produce telephone exchange equipments, each utilizing inductive coils of distinct resistive value per individule manufacturer. It is an imperative requirement in optimum operation, that inductive coil current be either zero, or a specifically rewired value at any and all times, and no other value between such limits. With the control transistor holding the series transistor at saturation, and using a metal oxide varistor as opposed to a silicon carbide one, the foregoing requirement is met.
Thus, it will be seen that circuitry causing inductive coil voltage to be dependent upon coil current, and in fact, enhancing variations in inductive coil current, was a disadvantage in the prior art. Furthermore, rated transistor gain being exceeded in such manner as to cause thermal failure is eliminated by use of the control transistor circuit in all operations wherein pulse repetition rate is above such level as 10 pps. and in all instances where current conditions affecting gain may cause thermal failure. The use of said control transistor circuit thus has far wider range than the use of a single transistor circuit.
Thus, not only are the foregoing specific problems of circuitry performance solved by the invention, but, in addition thereto, the invention is of adequate versatility to be useful regardless of resistive value of said inductive coil, thus enabling a telephone firm to standardize on a single product of this invention for updating of their existing equipments, and not being required to stock a plurality of specific and selected kits tailored to each of many kinds of inductive coils.